The present invention relates to an annular three-dimensional structure constituted by the regular criss-crossing of filiform elements distributed in a plurality of families of elements, each family being composed of elements of a same type occupying the same particular position within the structure.
A structure of this type can be used for example as reinforcement structure for a composite material included in the production of parts being symmetrical about an axis of revolution and needing to show good cohesion and uniform properties in every points of one circumference. A typical application of such a structure is the production of a carbon-carbon composite material (reinforcement structure made of filiform carbon elements densified by a carbon matrix) usable in the construction of nozzles or parts of nozzles for rocket engines.
Several types of three-dimensional structures have already been proposed.
For example it is known to produce annular structures consisting of a family of radial rectilinear elements, i.e. perpendicular to the axis of the structure, of a family of axial rectilinear elements, i.e. parallel to the axis of the structure, and of a family of incurved circumferential elements. By circumferential elements are meant here elements arranged in cylindrical sheets either in the form of separate circular spires situated at regular intervals in planes perpendicular to the axis, or in the form of helical windings of low pitch with spires whose inclination with respect to the planes perpendicular to the axis is as small as possible. Such a structure is described in French Patent Application No. 2,408,676.
It is also known to produce an annular structure constituted of a first family of incurved helical elements arranged in co-axial cylindrical sheets according to helical paths of identical pitch and direction, of a second family of incurved helical elements arranged in co-axial cylindrical sheets alternated with those of the first family according to helical paths of same pitch as the first family but of different direction, and of a family of radial rectilinear elements.
These two known annular structures composed of three families have a point in common in that they both comprise two families arranged in adjacent layers separated one from the other by separating surfaces traversed only by rectilinear elements of the radial element family. Said rectilinear elements cannot ensure a complete locking of the layers of the other two families. There is then for these layers a possibility of delamination, i.e. of decohesion in parallel to the radial elements. It is conceivable that such a possibility should be highly undesirable when one of the intended applications is the construction of parts requiring great cohesion.
In order to overcome this risk of decohesion, it has been proposed in U.S. Patent Application Ser. No. 19,571, to provide an annular structure constituted of four families of filiform elements, and in particular a structure of the type comprising first and second families of meridian rectilinear elements, oblique with respect to the axis of the structure and forming in each family, meridian sheets regularly distributed about the axis of the structure, and third and fourth families of helical elements forming, in each family, cylindrical sheets of the same axis as the structure and distributed regularly in radial direction, the inclination with respect to the axis of the elements of the first family being equal but opposite to that of the elements of the second family and the helical paths of the elements of the third family having the same pitch as those of the elements of the fourth family, but with an opposite orientation.
By oblique meridian element is meant here an element situated in a meridian plane and forming with the axis of the structure a different angle of 0.degree., 90.degree. and 180.degree..
The adjacent layers of two families are undoubtedly locked together by the elements of the other two families traversing the separation surfaces between said layers with different inclinations with respect to the said surfaces. Nonetheless, this four-family structure can only be achieved by following certain dispositions, which limits the possible applications of the structure.
It is therefore the object of the present invention to propose an annular three-dimensional structure presenting no risk of decohesion and being adaptable to satisfy special requirements relative for example to the arrangement of the elements or to the space distribution of the properties of the structure, whilst preserving a high value of reinforcement element volume content, i.e. keeping high the ratio of the real volume occupied by the elements of the structure to the apparent volume thereof.
This object is reached with a structure of the aforesaid type, comprising two families of oblique meridian elements and two families of helical elements, which structure is characterized in that it comprises at least a fifth family selected from the group constituted by a family of radial elements forming meridian sheets distributed regularly about the axis of the structure, a family of circumferential elements forming cylindrical sheets having the same axis as the structure and distributed regularly in radial direction and a family of axial elements forming meridian or cylindrical sheets distributed regularly in the structure.
The elements of each family constituting the structure form meridian or cylindrical sheets in which they are spaced out at regular intervals and have the same orientation.
The first and second family being composed of oblique meridian elements with equal but opposite inclination with respect to the axis of the structure, form a virtually symmetrical assembly with respect to meridian planes and to planes perpendicular to the axis. It is practically the same with the assembly constituted by the third and fourth family since these are composed of elements arranged in concentric cylindrical layers according to helical paths of identical pitch but of opposite orientation. Since the or each additional family is a family of radial, circumferential or axial elements, the structure, on the whole, is virtually symmetrical according to meridian planes and to planes perpendicular to the axis.
Another advantage of the structure according to the invention resides in the fact that the adjunction of a family of radial, circumferential or axial elements permits, depending on the need, to give certain mechanical or physical properties to the parts to be produced, due to the presence of reinforcement in one or more of the three main "directions" of the ring.
Also, due to the fact that the structure according to the invention comprises at least five different families, it is possible to obtain a balanced structure approaching isotropy whenever this is desired.
Finally, the number of families of elements which have different arrangements in the structure according to the invention cancels out the risk of decohesion.